Polymer particles with a diameter of about 0.01 to about 30 μm play an important role in many areas of plastics technology. Adhesives or coating compositions containing polymer particles with diameters in this range are frequently used in particular for bonding articles or for coating surfaces. Examples of the range of applications include decorative surface coating in the form of paint coatings or other coatings and the bonding of two surfaces having identical or different properties with adhesives of the type frequently encountered in the field of bonding.
The polymer particles mentioned are generally produced by polymerization of olefinically unsaturated monomers of which the properties can be modified by the polymerization of mixtures of various monomers, for example by co- or terpolymerization. The modified properties of a single polymer particle are generally transferred as a collective property to a surface coating or bond prepared with the aid of such polymer particles. Collective properties which are often subjected to an application-related modification include, for example, hardness, elasticity and—above all in adhesive applications—bond strength, open time or early adhesion.
Certain properties of the composition used to prepare the coating or the bond, for example the surface coating composition or the adhesive, are just as important. In this connection, viscosity, flow behavior or residual monomer content are often modified in dependence upon the desired application.
Inorganic or organic fillers are often added to a surface coating composition or an adhesive containing polymer particles to modify the above-mentioned properties of a surface coating or a bond. Inorganic fillers reduce the level of organic polymer material in the coating and thus contribute to greater hardness of the coating and to protection of the environment and raw materials through a lower content of organic polymers. Organic fillers are capable of imparting certain combined properties of the filler and polymer particles present therein, for example to a polymer dispersion.
Whereas mixtures of polymer particles and fillers frequently lead to dull, rough surfaces in the field of surface coating, their effect in adhesives is often that the strength and elasticity of adhesive bonds are greatly reduced. In addition, the presence of fillers in polymer dispersions often contributes to a reduction in early adhesive strength.
Such effects are drastically noticeable above all in the field of high-quality applications of surface coatings and bonds, such as high-gloss paints and bonds in the paper or furniture industry. For example, a significant reduction in ultimate adhesive strength on wood is often observed, accompanied by reduced early adhesion.
EP-A 0 392 065 relates to polymer-coated filler particles where a filler core is coated with a membrane of a hydrophobic polymer. To produce these polymer-coated particles, a monomer is polymerized in aqueous dispersion in the presence of filler particles and an amphiphilic polymer. The document in question does not contain any teaching on the production of filler-containing polymer dispersions in which the filler particles and the polymer particles are present substantially alongside one another.
DE-OS 22 43 687 relates to an adhesive, more particularly for the production of corrugated cardboard. Although this document does describe a polymer dispersion in which polymer particles and mineral particles are present together and does indicate size ranges for the particular particles, it does not contain any teaching as to the particle size ratio along the lines of the present specification.
Accordingly, the problem addressed by the present invention was to overcome the above-mentioned disadvantages of the mixtures of polymer particles and fillers known from the prior art. More particularly, the problem addressed by the present invention was to provide a filler-containing polymer dispersion which would have the advantages of the mixtures of polymer particles and fillers known from the prior art without any of their known disadvantages.
Accordingly, a particular problem addressed by the present invention was to provide a filler-containing polymer dispersion which could be used for the production of high-quality surface coatings or bonds.
Another problem addressed by the invention was to provide a filler-containing polymer dispersion which could be used as a high-strength adhesive, particularly on wood.
A further problem addressed by the invention was to provide a filler-containing polymer dispersion with a low residual monomer content.
Yet another problem addressed by the invention was to provide an adhesive which would have an adequate open time, high early adhesion and a low viscosity coupled with excellent flow behavior.
A final problem addressed by the invention was to provide a process for the production of such filler-containing polymer dispersions.